In an electrostatographic copy machine, an electrostatic latent image is formed on an element. That image is developed by the application of an oppositely charged toner to the element. The image-forming toner on the element is then transferred to a receiver where it is permanently fixed, typically by heat fusion. The transfer of the toner to the receiver is usually accomplished electrostatically by means of an electrostatic bias between the receiver and the element.
In order to produce copies of very high resolution and low granularity, it is necessary to use toner particles that have a very small particle size, i.e., less than about 8 micrometers. (Particle size herein refers to mean volume weighted diameter as measured by conventional diameter measuring devices such as a Coulter Multisizer, sold by Coulter, Inc. Mean volume weighted diameter is the sum of the mass of each particle times the diameter of a spherical particle of equal mass and density, divided by total particle mass.) However, it has been found that it is very difficult to electrostatically transfer such fine toner particles from the element to the receiver, especially when they are less than 6 micrometers in diameter. That is, fine toner particles frequently do not transfer from the element with reasonable efficiency. Moreover, those particles which do transfer frequently fail to transfer to a position on the receiver that is directly opposite their position on the element, but rather, under the influence of coulombic forces, tend to scatter, thus lowering the resolution of the transferred image and increasing the grain and mottle. Thus, high resolution images of low granularity require very small particles, however, images having high resolution and low granularity have not been attainable using electrostatically assisted transfer.
In order to avoid this problem, it has become necessary to transfer the toner from the element to the receiver by non-electrostatic processes. One such process is the thermally assisted transfer process where the receiver is heated, typically to about 60.degree. to about 90.degree. C., and is pressed against the toner particles on the element. The heated receiver sinters the toner particles causing them to stick to each other and to the receiver thereby effecting the transfer of the toner from the element to the receiver. The element and receiver are then separated and the toner image is fixed, e.g., thermally fused to the receiver. For details, see copending application Ser. No. 230,394, U.S. Pat. No. 4,927,727, titled "Thermally Assisted Transfer of Small Electrostatographic Toner Particles" filed Aug. 9, 1988.
While the thermally assisted transfer process does transfer very small particles without the scattering that occurs with electrostatic transfer processes, it is sometimes difficult to transfer all of the toner particles by this process. The toner particles that are directly on the element often experience a greater attractive force to the element than they do to the receiver and to other toner particles that are stacked above them, and the heat from the receiver may have diminished to such an extent by the time it reaches the toner particles next to the element that it does not sinter them. As a result, the toner particles that are in contact with the element may not transfer. Attempts to solve this problem by coating the element with a release agent have not proven to be successful because the process tends to wipe the release agent off the element into the developer which degrades both the developer and the development process. Moreover, because the process tends to wipe the release agent off the element, the application of additional release agent to the element is periodically required in order to prevent the toner particles from adhering to the element during transfer.
An alternative approach to removing all of the toner particles from the element is to use a receiver that has been coated with a thermoplastic polymer. During transfer, the toner particles adhere to or become partially or slightly embedded in the thermoplastic polymer coating and are thereby removed from the element. However, it has been found that many thermoplastics that are capable of removing all of the toner particles also tend to adhere to the element. This, of course, not only seriously impairs image quality but it may also damage both the element and the receiver. Moreover, until now, it has not been possible to predict with any degree of certainty which thermoplastic polymers will remove all of the toner particles from the element without sticking to the element during transfer and subsequent separation of the receiver from the element and which ones will not.
In copending U.S. application Ser. No. 345,160 U.S. Pat. No. 4,968,578, entitled "Method of Non-Electrostatically Transferring Toner" filed Apr. 28, 1989, which is a continuation-in-part in of U.S. application Ser. No. 230,381 abandoned, entitled "Improved Method of Non-Electrostatically Transferring Toner" filed Aug. 9, 1988, it is disclosed that if such small sized toner particles are transferred to a receiver formed of a substrate or a support which has been coated with a thermoplastic polymer having a layer of a release agent on the thermoplastic polymer coating and the receiver is heated above the Tg of the thermoplastic polymer during transfer, the release agent will prevent the thermoplastic polymer coating from adhering to the element but it will not prevent the toner from transferring to the thermoplastic polymer coating on the receiver and virtually all of the toner will transfer to the receiver. This constitutes a significant advancement in the art because it is now possible not only to obtain the high image quality that was not previously attainable when very small toner particles were transferred electrostatically but, in addition, the problem of incomplete transfer is avoided. In addition, several other advantages are provided by this process. One such advantage is that copies made by this process can be given a more uniform gloss because all of the receiver is coated with a thermoplastic polymer, (which can be made glossy) while, in receivers that are not coated with a thermoplastic polymer, only those portions of the receiver that are covered with toner can be made glossy and the level of gloss varies with the amount of toner. Another advantage of the process is that when the toner is fixed, it is driven more or less intact into the thermoplastic polymer coating rather than being flattened and spread out over the receiver. This also results in a higher resolution image and less grain. Finally, in images made using this process, light tends to reflect from behind the embedded toner particles that are in the thermoplastic layer which causes the light to diffuse more making the image appear less grainy.
For all of the benefits and advantages provided by this process, however, the application of a release agent to the thermoplastic polymer coating on the receiver in order to prevent the thermoplastic polymer coating from adhering to the surface of the element during transfer and subsequent separation of the receiver from the element creates several problems. One such problem is that the release agent tends to transfer to and build up on the element or photoconductor thereby degrading image quality and causing potential damage to both the element and the receiver. Another problem is that the release agent tends to allow the thermoplastic polymer coating to separate from the support or substrate, especially during or after finishing, due to a reduction in the adhesion strength of the thermoplastic polymer coating to the receiver support caused by the tendency of the release agent, which has a lower surface energy than the thermoplastic polymer coating and hence a lesser predilection to adhere to the receiver support than the thermoplastic polymer coating, to migrate through the thermoplastic polymer coating to the interfacial region between the thermoplastic polymer coating and the support and to cause the thermoplastic polymer coating to separate from the support. It has also been found that the release agent reduces the gloss of the finished image. Finally, the addition of a release agent to the thermoplastic polymer coating adds to the overall cost of the process.
Accordingly, it would be desirable to be able to provide a thermally assisted transfer process for transferring dry toner particles having a particle size of less than 8 micrometers from an element to a receiver in which a thermoplastic polymer coated receiver is utilized such that all of the benefits and advantages afforded by the use of a thermoplastic polymer coated receiver in a thermally assisted transfer process are retained, including the transfer of virtually all of the toner particles from the element to the receiver, but one which does not require the use of a coating or layer of a release agent on the thermoplastic polymer coating on the receiver substrate (or the element) in order to prevent the receiver from adhering to the element during transfer and subsequent separation from the element. The present invention provides such a process.